This invention relates generally to a fluidized-bed reactor in which waste water or other liquid to be processed is conducted upwardly through a bed of small carrier particles which in the course of operation are enlarged by the growth of cellular material thereon, and more particularly to a system having a broad operating range for controlling the accumulation of such material in a fluidized-bed reactor to prevent excessive growth thereof.
The invention will be described in connection with reactors for extracting nitrogen compounds such as nitrate or ammonia from waste water by means of a fluidized bed of solid particles serving as carriers for micro-organisms grown thereon. These reactors are preferably of the type disclosed in the Jeris et al. U.S. Pat. No. 3,846,289 and in the Jeris U.S. Pat. No. 4,009,099, whose entire disclosures are incorporated herein by reference.
It is to be understood, however, that a system in accordance with the invention is also applicable to other forms of fluidized, expanded or moving bed reactors in which the need exists to control the build-up of bacteria and other types of cellular material on the carrier particles to prevent excessive growth thereof. Thus the invention is useful in connection with the fluidized-bed reactors of the type disclosed in U.S. Pat. No. 4,032,407 and in the Jeris U.S. Pat. No. 4,009,098 and 4,009,105.
It is now recognized that the existence of nitrogen compounds in waste water represents a serious threat to ecological balances existing in nature. Accelerated eutrophication of lakes and streams is often caused by feeding conventionally-treated waste effluent into surface waters. While such treatment is designed to remove solids and oxygen-demanding organic material, they do not extract from these wastes the substantial quantities of ammonia and nitrate ions which are contained therein and which promote the production of algae.
Aside from being a major nutrient to algae growth, nitrogen in the form of ammonia is toxic to aquatic life and can react with a chlorine disinfectant to form chloramines which are carcinogenic. Hence there is a need for waste water treatment that effects almost complete denitrification; that is, the conversion of nitrate or nitrite compounds to non-polluting elemental nitrogen gas prior to the release of the wastes to surface waters.
The fluidized-bed reactors disclosed in the above-identified Jeris et al. and Jeris patents accomplish denitrification by a biological process in which bacteria act to reduce nitrite or nitrate constituents in the influent waste stream into harmless nitrogen gas. This process is carried out in a fluidized-bed reactor in which the waste water to be treated passes upwardly through a bed of small particles, such as activated carbon or sand, at a velocity sufficient to cause motion or fluidization of all of the medium which then functions as a carrier or support surface for the growth of bacteria.
The use of small sand or other particles provides a vast surface area on which the bacteria can flourish and grow, thereby making it possible to remove a considerable amount of contaminants from the waste water in a relatively small reactor volume. Fluidization of the medium augments the effective surface area, compared to that of packed beds, and it minimizes operational problems such as the clogging and head loss encountered in packed beds.
As waste water containing nitrogen in the form of ammonia or nitrate is passed through a fluidized bed, bacterial growth is accelerated and the size of the particles undergoes enlargement. If this growth is unchecked, the bed particles become enlarged to a degree resulting in agglomeration, thereby reducing the biological surface area per unit volume of the reactor and the efficiency of the reactor column. Moreover, as the particles swell, they are reduced in specific gravity and thereby acquire a tendency to float and to be carried away from the bed. Also, when the particles are excessively enlarged, they are prone to entrap or become attached to gas bubbles. This further reduces the specific gravity of the particles and the tendency of the particles to be carried away from the bed.
The primary concern of the present invention is with the removal of excess cellular material or bacterial growth on the particles of a fluidized-bed reactor in the course of operation, thereby obviating the tendency of the particles to be carried away in the process effluent. The term "excess cellular material" as used herein refers to the amount of material attached to the particulate carrier beyond that needed for the normal operating of the reactor. In a fluidized-bed reactor for denitrification, sufficient growth in the form of a thin layer of bacteria must be retained on the particles in order to maintain the efficiency of the process. Hence a system which so abrades or shears the particles as to remove all bacterial growth is destructive of the process.
Another concern of the invention is with the control of the thickness of the bio-mass layer on the carrier particles so that an optimum film thickness can be maintained in the biological reactor.
Various techniques have heretofore been proposed to prevent the accumulation of excess cellular material on the carrier particles in a fluidized-bed reactor. One such technique is disclosed in the Jeris U.S. Pat. No. 4,009,099 wherein the bacterial growth on the particles is monitored as a function of bed expansion. This is accomplished by an optical device or other form of solids sensor, such that when bed expansion reaches a given height to activate the sensor, the bed particles are regenerated by abrasion to remove excessive cellular material.
This is effected mechanically by a stirrer at the top of the column in the form of sharp rotating blades or other means. The partially stripped carrier particles acted upon by the stirrer settle back into the fluidized bed, whereas the sheared-off excess cellular material which has greater buoyancy than the carrier particles is carried away in the effluent process stream.
Since in this prior Jeris arrangement, the output of the reactor includes the excess cellular material, it is necessary to use a clarifier or other solids-separation means to remove the sheared solids from the process effluent.
Another approach heretofore taken to control the production of sludge is to permit the growth-covered particles to flow out with the process effluent into a settling tank which separates these particles from the process effluent. The excess growth is then mechanically sheared from the carrier particles, and the mixture of sheared sludge and particles is returned to the fluidized bed. In this technique, a clarifier or other solids separation unit must be used in conjunction with the settling tank to remove the sheared sludge from the process effluent. Alternatively, the sheared growth can be separated from the carrier particles in the return path between the settling tank and the fluidized bed, thereby eliminating the need for a clarifier in the output of the reactor, but requiring instead a separator unit in the return path.
Still another effective approach in current use for removing excess sludge is by means of a vibrating screen. In this technique, the growth-covered particles are pumped from the fluidized bed to the vibrating screen, the pumping action serving to agitate the particles and to shear excess growth therefrom and the vibrating screen functioning to separate the sheared growth from the carrier particles. These particles are then returned to the fluidized bed, whereas the sheared growth is wasted. Alternatively, shearing may be effected by means other than a pump.
The use of a vibrating screen in a growth control system has one important advantage, for it obviates the need for a clarifier in the output of the reactor. On the other hand, a vibrating screen is subject to plugging, and this dictates the use of a washing spray to keep the screen free. This is a serious drawback; for the spray dilutes the concentration of the waste sludge which must be further processed.
Moreover, during the operation of a vibrating screen, some sheared solids remain loosely attached to the carrier particles, and when the carrier particles are returned to the fluidized bed, these solids are brought along and escape into the process effluent, thereby degrading the quality of the effluent when the screen is operative. Furthermore, a vibrating screen, which is a fairly expensive and sophisticated unit, has inherent practical limitations with respect to the amount of liquid it can handle.
In calculating the overall cost of installing and operating an excess growth control system, one must not only take into account the amount of energy that is necessary to shear excess growth from the carrier particles but also the fact this shearing action directly affects the dewaterability of the resultant sludge. In order to dispose of this sludge, it must first be dewatered. If, for example, the abrading technique for shearing the growth tends to dissect the sludge into fine pieces which are then suspended in the water and are slow to settle, dewaterability becomes more difficult.
The nature of the pollutant being removed from the liquid and the type of fluidized-bed process employed for this purpose determines the type of biological organisms which predominate in the treatment system therefor. For a given system, there is an optimum amount of energy which must be imparted to the excess growth shearing means to attain the most dewaterable sludge.
Inasmuch as the handling and disposal of the sludge in a biological treatment facility can represent a substantial percentage of the overall cost of treatment, running as high as 40%, the dewaterability of the excess sludge production directly affects this cost and cannot, therefore, be disregarded; for the more easily dewaterable the sludge, the less expensive is the sludge-handling procedure.
Thus while various techniques and systems based thereon have heretofore been provided to control excess growth in a fluidized-bed reactor, all of these known techniques have entailed clarifiers and other expedients which add substantially to the cost of installing and operating the control systems in a manner yielding an effluent free of sheared material.
In our above-identified copending application, whose entire disclosure is incorporated herein by reference, there is disclosed a highly-efficient system for controlling the growth of cellular material on the carrier particles of a fluidized-bed reactor, the energy required to shear growth from the carrier particles being governed to produce a sludge having optimum dewaterability characteristics.
In a system of the type disclosed in my copending application for preventing the accumulation of excessive cellular material in a fluidized-bed reactor wherein waste water or other liquid to be processed is passed upwardly at a velocity conducive to fluidization through a bed of particles which function as a carrier for the growth of the material, a head of effluent is developed above the bed, the effluent being discharged from the reactor through a clear-effluent port.
Included in the system is an open-ended separator column whose low end extends toward the fluidized bed and whose high end extends above the effluent head or is sealed and located below the surface of the effluent head. In the course of reactor operation, the cellular material on the particles continues to build up, causing the bed to expand until its level reaches an alarm point indicative of excessive growth. This alarm point is sensed to activate an agitator arrangement which effects shearing of the excess growth from the carrier particles to generate in the separation column a mixture of sheared material and partially-stripped carrier particles.
The separator column is provided with a draw-off port somewhat below the surface of the effluent head. The exit flow rate of the draw-off port is adjusted so that the upward flow velocity in the separator column is lower than the settling velocity of the carrier particles in the mixture, but higher that that of the sheared material. As a consequence, the sheared material is washed away through the draw-off port, whereas the partially-stripped carrier particles fall back into the fluidized bed. This excess growth removal continues until the level of the bed falls to a predetermined safety point below the alarm point when the activity is discontinued to complete the cycle which is not repeated until the bed again expands to reach the alarm point.
Because the sheared growth is confined to the separator column, none of this material can enter the process effluent stream; hence there may be no need for a clarifier in the output line of the fluidized-bed reactor as in prior control systems to provide a clear effluent.
The practical difficulty experienced with a system of the type disclosed in our copending application is that the operating range of liquid velocity through the separator column is limited. If, therefore, in a given installation, the upward velocity is slightly higher than a predetermined acceptable level, the sand or carrier particles which have been partially sheared may be carried out with the sheared material and pass through the sludge discharge port instead of settling back into the bed.
On the other hand, if the liquid velocity is slightly lower than the predetermined acceptable level, the sludge may back up into the fluidized bed reactor. Since the optimum velocity may vary from plant to plant, this factor creates difficulties in designing the unit for different bacterial processes.